Direct pigment photographic printing is a dichromate based photographic printing process. Literature references to this type of process are somewhat inconsistent with the printing being referred to as Artique printing, Fresson printing, direct carbon printing and in some instances, simply a miscellaneous non-silver printing process. Direct pigment printing historically evolved from both gum printing and carbon printing processes. Neither process has extensive general commercial value today due to the considerable technical advancements in photography printing as well as the inherent labor intensity of both processes. However, gum printing, carbon printing, and direct pigment printing are all commercially practiced by hobbyists and commercial artists due to the ability to create artistic prints which exhibit special beauty and permanence along with the ability to simulate historical and archival prints. Nostalgic or historical prints are sold as prints, framed pictures, post cards, greeting cards, and similar commercially printed items.
Gum printing utilizes a gum arabic solution to which a dichromate or other light sensitive dye is added along with pigment. The coating is typically brush coated on paper, allowed to dry and exposed to actinic light through a standard continuous tone negative. The gum arabic becomes water insoluble in proportion to the intensity of light exposure transmitted through the negative. The print is developed by floating face down in a tray of cool water whereby soluble gum arabic slowly dissolves leaving an image that consists of the undissolved gum arabic and pigment. Only a limited amount of pigment can be added to the gum arabic solution before excess pigment begins to block out the highlights of the image due to pigment falling out of suspension and staining the paper base. Due to the limited amount of pigment that can be utilized in the gum arabic solution, the resulting image lacks overall intensity. To overcome this problem, gum prints are commonly made through repeated printings of the same negative. A typical gum print is made by repeatedly coating, exposing and developing the same negative until sufficient image intensity is achieved to produce a finished print.
Carbon printing utilizes a sheet of paper coated with a layer of pigmented gelatin commonly called carbon tissue. The carbon tissue is sensitized with a dichromate or alternate light sensitive dye, dried, and then exposed to actinic light through a continuous tone negative. The gelatin becomes insoluble in proportion to light exposure. The insolubility is a variable gradient where insolubility decreases from the surface of the gelatin layer downward toward the paper. If the image is developed on the carbon tissue's paper support, the highlights and a portion of the middle tones of the image are lost. To overcome this problem, the carbon tissue can be dipped or wetted in cool water and followed by the gelatin side contacted with another sheet of paper containing a thin surface coating of hardened gelatin to permit transferring of the image to the gelatin surface. The paper having a hardened gelatin layer is called transfer paper. The two sheets of paper are kept in contact for about 20 minutes and then soaked in water for several minutes until the two sheets of paper can be pulled apart. This procedure transfers the image with all of its tones to the second sheet of paper. This second sheet of paper is then developed by pouring warm water over the image surface to remove the soluble gelatin and pigment and thereby leave the final printed image.
Both processes, although laborious as described above, have the ability to produce photographic prints which can be as permanent as artist water color paintings. Both processes can utilize mineral pigments or inorganic colorants of known resistance to fading or color changes. In either procedure, all of the residual light sensitive chemical can be removed from the paper through adequate washing, and the final print amounts to a pigmented layer of either gelatin or gum arabic.
Direct pigment printing is a specific type of carbon printing and generally provides a method of photographic printing which provides an image which is softer looking than standard silver printing. Due to the nature of the direct pigment printing process, the image is composed of ground mineral pigments selected to provide prints that are light resistant and archival in appearance as water color or art paintings which are characteristically stable with passage of time. Ordinarily, ground mineral pigments are preferred since organic dyes are not ordinarily permanent. Direct pigment prints can be made as monochrome (black and white) or in color or in multiple colors through the use of multiple pass printing. Conventional photographic prints, especially color prints, fade and change colors as time passes due to interaction of the print surface with the environment including, for instance, UV light degradation of the organic dyes and other chemicals used in the printing process. A so-called Fresson print named after a Frenchman who popularized direct pigment printing in the late 1800's reportedly is based on a process where pigment mixed with a colloid is spread on a sheet of paper which is sensitized to light by dipping the colloid covered paper in a dichromate salt aqueous solution. When a contact print is made on this paper by actinic light through a negative the light action causes the sensitized colloid to become insoluble to water in proportion to varying degrees according to light exposure and the negative densities. Exposure to light causes the colloid to become insoluble in proportion to the light passed through the negative. Pouring a slurry mixture of water and sawdust over the exposed print dissolves and wears away the more soluble sections or less exposed areas of the negative, thus leaving a pigmented positive print. In essence, development of the print can be achieved by mild abrasion due to the sawdust/water slurry removing pigment by friction. More pigment is removed from less hardened colloid areas (highlights) while less pigment is removed from the more hardened colloid areas (shadows) and unexposed colloid and pigment areas merely wash away. Fresson prints reputably are the most beautiful and the most acclaimed direct pigment prints. Published attempts to produce Fresson prints either do not work or produce poor imitation Fresson prints. A major problem with direct carbon printing processes is that continuous tones are difficult to achieve. Dark tones and middle tones can be obtained but highlight tones in the negatives are not reproduced in the print. Typically, highlight tones are seen as blurred non-definitive grays with no sharp contrasts between grayish background or shadows.
It now has been found that substantially improved direct pigment prints can be obtained in accordance with the process of this invention to produce prints exhibiting sharp continuous tones wherein definitive dark tones and light tones are obtained along with definitive middle tones to produce clear, definitive, continuous tone prints. In accordance with this invention, direct pigment printing paper can be produced from sized paper (size applied to seal fibers in the paper) by applying a first coating containing a heated mixture of gelatin and gum arabic followed by applying a second coating containing a heated mixture of gelatin, gum arabic, and suitable black, white, or colored pigment. Upon exposure to light followed by development processing, a sharp, continuous tone pigment print is produced. It has been found that if both the first coating and the second pigmented coating are based on a combination of gelatin and gum arabic, the resulting prints exhibit excellent continuous tones with definitive middle tones and clear highlights. The process of this invention produces a finished print with one printing in contrast with several printings in the gum process and, further, produces a finished print without the need to transfer the image as in the carbon printing process. None of the prior art processes suggest a combination of gelatin and gum arabic and, in fact, such mixtures are normally incompatible. This discovery was surprising since normally unheated mixtures of gum arabic and gelatin are incompatible.
A mixture of gum arabic and gelatin permit a photographic image to be made directly on a substrate without the need for multiple printings to achieve definitive tonal range as in gum printing and without the need for image transferring as in carbon printing. By applying the coating mixtures in multiple layers with varying gum arabic to gelatin ratios with the image black, white and/or color pigmentation in the top coating, control of the highlight and shadow tones alike can be advantageously achieved. Although colored pigments are added in the top coating, white pigments can be added in intermediate coatings to increase highlight (white) areas in the print. A pigmented mixture of gum arabic and gelatin provides the ability to reproduce a monochrome photographic negative or positive transparency image as a full tonal range photographic printed image without multiple printing or image transfer. The process is particularly useful for producing archival and artistic photographic prints. These and other advantages of this invention will become more apparent by referring to the detailed description and illustrative examples.